MEMBRANE STRUCTURE AND FUNCTION Membrane transport “Got to get it there” Chapter 7 Continued

Objectives Understand what factors influence membrane permeability Understand the difference between passive and active transport Be able to discuss the processes of: Diffusion, Osmosis, Facilitated diffusion Pumps, Cotransport, Exocytosis and Endocytosis

Why do materials move? Molecules are in motion The motion of molecules increases the entropy of the system (less order) Molecular collisions result in molecules being moved along a gradient (concentration gradient or free energy gradient) Through random molecular collisions directional motion can be accomplished

Membrane Permeability Hydrophobic middle of the bilayer inhibits the passage of ions and polar molecules that are hydrophilic Transport proteins may assist ions and polar molecules across the membrane Can cross lipid bilayer = nonpolar, small uncharged polar molecules Cannot cross lipid bilayer = large polar and charged molecules

Transport Mechanisms Passive Transport –moves molecules along a concentration gradient –no cellular energy required Active transport –moves molecules against a concentration gradient –requires cellular energy The movement of materials across membranes takes many routes but the mechanisms are categorized as either passive or active transport mechanisms

Passive Transport Mechanism Simple Diffusion: the movement of a substance from higher concentration to lesser concentration Simple diffusion occurs across the lipid bilayer The bilayer is selectively permeable as not everything can get across

Specific examples of diffusion: Osmosis Osmosis: the diffusion of water (solvent) across a membrane –influenced by total solute concentration Osmotic pressure: pressure exerted on a membrane due to an imbalance of solute between the inside and outside of the membrane Water always moves toward the side with a greater concentration of solute

Cells respond to osmosis, so what? Tonicity: the ability of a solution to move water –Hypertonic: Greater ability to move H 2 O; gains water –Hypotonic: Lesser ability to move H 2 O; loses water –Isotonic: equal ability to move H 2 O; no net water movement

The Importance of Osmoregulation Living things must balance water uptake and loss If cells lose water they crenate If cells gain water they lyse Expulsion vacuole Turgor pressure

Transport Proteins: Facilitated Diffusion Via Channel Proteins Involves transport proteins moving a solute along a concentration gradient May be specific May be saturated (can only work so fast) or inhibited Passive mechanism

Transport Proteins: Facilitated Diffusion Via Carrier Proteins Molecule causes a controlled denaturation resulting in a molecule being transported May be specific May be saturated or inhibited Protein assists the process of diffusion; passive mechanism

Regulation of Facilitated Diffusion Some transport proteins are regulated by chemical or electrical stimuli Usually these proteins are permanent channels that are opened or closed via other proteins (gated channels)

Active Transport: Pumps Moves solute uphill and requires energy Always requires carrier proteins Major factor that allows the cell to regulate the concentration of solute within the cell May result in an imbalance of solute across a membrane that the cell can utilize

Kinds of Pumps Na + /K + H + Ca 2+ Some pumps create electrical differences across a membrane (electrogenic pumps)

Electrochemical Gradient The difference in voltage across a membrane resulting from electrogenic pumps is called membrane potential This electrical force affects the transport of charged solutes Cations are favored because interior of cell is usually negative compared to the outside Resulting Electrochemical Gradient affects ion transport: electrical = membrane potential chemical = concentration gradient

Cotransport (secondary active transport) Two solutes transported at one time via a transport protein –one with a gradient –one against a gradient Solute moving with the gradient does so because of an earlier active transport event (pump)

Exocytosis and Endocytosis Exocytosis involve the movement of macromolecules out of the cell by the fusion of membrane bound vesicles to the plasma membrane Endocytosis involves the movement of macromolecules into the cell by the pinching of the plasma membrane into membrane bound vesicles –Phagocytosis –Pinocytosis –Receptor-mediated pinocytosis

Endocytosis Phagocytosis: ingestion of large particle Pinocytosis: ingestion of small mixed solutes Receptor-mediated pinocytosis: ingestion of specific solutes (ligands) with the aid of binding proteins; areas called coated pits